The American shale boom will begin to taper sometime in the mid-2020s, after which point the world will fall back to depending on the Middle East more than ever to meet energy needs, according to the International Energy Agency.

But this time, the traditional Gulf players may not be ready to shoulder the burden.

In its latest outlook, the IEA projects that while total spending on oil drilling will remain fairly constant (at $500 billion a year) through 2035, "there will be a noticeable shift in the location of this investment over the coming decades."

That shift will be away from North America — where the most accessible plays will dry up and the expense required to extract unconventional oil will rise — and back toward traditional petro states, where the cost of extraction will remain relatively cheaper and the resource base more extensive.

"Gradual depletion of the most accessible reserves forces companies to move to develop more challenging fields," the agency says. "...Although offset in part by technology learning, this puts pressure on upstream costs and underpins an oil price that rises to reach $128/barrel in real terms by 2035."

Here's the chart, showing 13% of the total upstream oil investment in OPEC countries of the Middle East accounts for a third of the new resources developed to 2035, whereas tight oil in North America accounts for 13% of investment but for just 6% of new resources.

"Meeting long-term oil demand growth depends increasingly on the Middle East, once the current rise in non-OPEC supply starts to run out of steam in the 2020s," the agency says.

But the Middle East, in the form of OPEC, may miss its cue to jump in to the breach, the agency says, because the American boom has slowed oil price growth and diverted OPEC resources to non-traditional projects. As a result, spending on oil development among OPEC players will be constrained.

"The result would be tighter and more volatile oil markets, with an average price $15/barrel higher in 2025," the agency says.

This outcome is not a total certainty. The IEA notes that the "learning" effect in North American drilling — whereby the longer a play is developed, the more efficient drilling in that play becomes — could extend beyond their original taper date.

And the fact that the price of oil will increase, on average, less than 1% a year to 2035 further shows the shale boom has created a huge cushion for the rest of the world.

But it looks like, one way or another, we'll be returning to the way things once were.